1. Field of the Invention
This invention relates to a method of chemical treating aluminum cathodes used in zinc electrowinning to remove recrystallized particles of zinc sulphate salts, and other impurities which may affect the zinc deposition process.
2. Description of the Related Art
During the zinc electrowinning process, zinc is plated onto aluminum cathodes while oxygen is evolved at the Pb-Ag anode. The zinc deposit is stripped from the cathode after a predetermined plating period which depends on the mode of operation of the zinc plant. The stripping process consists of removing the aluminum cathodes from the cells, followed by either manually or automatically stripping the zinc deposit. When the cathodes are removed from the cells and stripped, some electrolyte remains on the surface of the cathodes despite the water rinsing treatment. As a result, the cathodes especially in the area close to the edges of the cathode, is corroded at a varying degree, depending on the quantity and concentration of the acid in contact with the cathode. Evaporation of the electrolyte is also observed at the surface of cathode, resulting in precipitation of insoluble zinc sulphate salts and other impurities causing an increase in the corrosion rate of the aluminum cathode. The overall effect of this corrosion attack can be seen on the smoothness of the aluminum cathode, i.e., patches of rough areas appear at times on the surface of the aluminum. Because of the unevenness in the surface of the cathode and of the presence of impurities, the zinc deposition process is affected resulting in formation of rough zinc deposits. Usually, these areas are seen as "puffed" sections of the deposits which, because of their closer proximity to the anode, tend to affect the current distribution in the electrolysis cell. As the zinc electrowinning process is sensitive to variations in current density, the uneven current distribution observed with puffed zinc deposits, causes a decrease in the current efficiency of zinc deposition. Under these conditions, higher corrosion rates of the Pb-Ag anode are observed which result in an increase in the Pb content of the zinc deposits.
Another effect of the impurities on the surface of the aluminum cathode is the formation of pinholes on the zinc deposit. This also results in lower current efficiency of zinc deposition.
A known method of preventing the occurrence of puffed zinc deposits consists of mechanically or manually buffing the aluminum cathodes using metal or plastic brushes. Mechanical buffing is carried out using automated machines which apply a scrubbing action at the surface of the cathode. As a result the surface of the cathode is maintained free of deposited impurities. However, due to the presence of edge strips located at the sides and bottom of the aluminum cathode to prevent electrodeposition of zinc on the sides of the cathode and facilitate the stripping of the deposits, the mechanical buffing machines are not efficient in treating the entire surface of the cathode. Furthermore, mechanical or manual buffing of the affected cathodes does not completely remove the deposited impurities, and insoluble zinc sulphate salts from the surface of the electrode as the treated areas become affected after about three weeks necessitating rebuffing of the electrode.
To facilitate removal of impurities and insoluble zinc sulphate salts from an aluminum cathode used in zinc electrowinning a chemical treatment has been developed.